Conventionally, in an automobile, a heater having a heating resistor which generates heat upon supply of electricity thereto is used, in combination with an energization control apparatus for performing energization control for the heater, in order to assist startup of an engine, stably operate the engine, or heat the compartment of the automobile. Further, a widely used heating resistor has a positive correlation between temperature and resistance such that resistance increases with temperature. Examples of known schemes for controlling supply of electricity to a heater having such a heating resistor include a constant power control scheme and a resistance control scheme.
In the constant power control scheme, the electric power supplied to the heating resistor is obtained from voltage applied to the heating resistor and current flowing therethrough, and electricity is supplied to the heater such that a cumulative electric energy obtained through integration of the electric power becomes equal to a predetermined electric energy. When constant power control is performed, the heating resistor generates heat in proportion to the supplied electric energy. Thus, the temperature of the heating resistor can be elevated to a predetermined temperature through supply of a certain amount of electric energy. Therefore, the temperature of the heating resistor can be readily managed. This is because the heat generation amount (i.e., temperature) of the heating resistor greatly depends on the quality of the material of the heating resistor, and the quality of the material of the heating resistor can be readily made uniform industrially. The constant power control scheme is suitable in particular for prevention of excessive temperature increase at the beginning of supply of electricity to the heating resistor. However, maintaining the temperature of the heating resistor is difficult when the heating resistor is thermally influenced from the outside; e.g., when the heating resistor is cooled by a disturbance.
Meanwhile, in the resistance control scheme, by taking advantage of the positive correlation between the temperature and resistance of the heating resistor, the supply of electricity to the heating resistor is controlled such that the resistance of the heating resistor approaches a target resistance corresponding to a temperature set as a temperature increasing target. The resistance control scheme is advantageous in that, even when the heating resistor is influenced by a temperature change caused by a disturbance, the temperature of the heating resistor can be readily maintained constant. However, even when heating resistors are formed of the same material of the same quality, variations in properties may arise due to slight changes in cross sectional area and/or density of the heating resistors within the tolerance of the products. Therefore, even among heating resistors of the same model number, a difference (variation) arises in the correlation between temperature and resistance because of individual variations in properties.
In view of the foregoing, a glow plug energization control apparatus used with, for example, a diesel engine performs constant power control for a glow plug at the time of startup of the engine at which fluctuations of disturbances are small, to thereby elevate the temperature of a heating resistor (a resistance heating heater) to a target temperature. After having elevated the temperature, the control apparatus switches its control mode from constant power control to resistance control so as to maintain the resistance of the heating resistor at that time, to thereby maintain the temperature of the heating resistor at the target temperature (see, for example, Patent Document 1).
Incidentally, in the case where the correlation between temperature and resistance is corrected (calibrated) for an individual heating resistor, the correlation between temperature and resistance can be made constant irrespective of individual variations in properties. That is, since a resistance of a heating resistor corresponding to a target temperature is univocally determined, resistance control can be readily performed. Since the resistance of the heating resistor changes due to deterioration with time, if such calibration is performed every time an engine is operated; for example, during pre-heating of a glow plug (during a temperature increasing operation for causing the temperature of the heating resistor to approach the target temperature), the resistance control can be performed accurately after the temperature increasing operation.
However, when the engine is cranked (started) in the middle of the pre-heating of the glow plug; i.e., in the middle of the calibration, a disturbance, such a swirl within the engine, injection of fuel, or the like, arises, and the heating resistor is partially cooled, whereby the accuracy of the calibration may drop. Further, in the case of a generally employed heating resistor, change in resistance with deterioration with time does not become large until the deterioration progresses to a certain degree. Therefore, during a period in which the influence of the deterioration of the heating resistor is small, the correlation calibrated during a period in which the engine is not cranked can be used until the glow plug is exchanged with a new one; that is, until the heating resistor is replaced with another one. In order to allow such an operation, the exchange of the glow plug must be reported to an energization control apparatus (GCU) for the glow plug. Therefore, when the glow plug is exchanged with a new one, an operator reports the exchange of the glow plug to the GCU by means of, for example, operating a switch, so as to cause the GCU to discard the calibrated correlation for the old glow plug and perform calibration for the new glow plug.